Question: In physics class, the girl to boy ratio is $2$ to $9$. If there are a total of $55$ students, how many girls are there?
Answer: A ratio of $2$ girls to $9$ boys means that a set of $11$ students will have $2$ girls and $9$ boys. A class of $55$ students has $5$ sets of $11$ students. Because we know that there are $2$ girls in each set of $11$ students, the class must have $5$ groups of $2$ girls each. There is a total of $10$ girls in physics class.